One or more embodiments of the present invention relate to an AC-AC converter device.
AC-DC-AC converters are used for line conditioning by converting an AC input power, for example the mains, to a controlled AC output power supplied to a load. The load is typically a load requiring a reliable AC voltage and a reliable frequency, such as IT equipment for critical mission applications. The DC voltage can be provided from a battery or any other type of energy storage. The DC voltage can also originate from a solar panel or a wind generator provided with a suitable energy conversion module. AC-input could be the utility AC-line voltage and the output could be any AC-load.
An AC-AC converter typically consists of an AC-DC converter and a DC-AC converter. These two converters are linked together by a DC voltage which is filtered and kept constant over the line period by a large amount of capacitance or any given energy storage device. The DC-voltage is required to be larger than the maximum peak value of the positive and negative half cycle of the line voltage. In an UPS application, it is probably required that the neutral line is common for the load and the line voltage and the load voltage to be in phase with line voltage. One example is shown in “Space vector Modulation for Single Phase On-Line Three-Leg UPS”, Pinheiro et al, 2000 IEEE. One popular technique to achieve this is to have a pair of line frequency commutated switches connected in a half bridge configuration connecting either the negative branch of the intermediate DC-voltage to the neutral during positive half-period of the line voltage, and connect the positive branch of the intermediate DC-voltage to the neutral during negative half-period of the line voltage. The input AC/DC converter acts like a boost regulator, stepping the line input AC voltage to a higher DC-voltage, while controlling the input current to be sinusoidal and the DC voltage to be constant. The output DC/AC converter acts like a step-down regulator, controlling the output AC voltage to be sinusoidal and in phase with the input voltage. A typical three leg single phase UPS configuration is shown in FIG. 1. The legs are indicated by dashed boxes Bin, Bout, Bc.
However there is a considerable drawback with this type of circuit since the center leg CL of the center half bridge Bc is common for the in/out rectifier stage (box Bin) and the output inverter stage (box Bout). The timing for controlling the switches connected to the center leg needs to be performed with extreme high precision because no phase shift between line and load voltage can be tolerated.
The prior art AC-AC converter device shown in FIG. 2 is known from “A study of the High Performance Single Phase UPS”, Tamotsu Ninomiya et al, 0-7803-4489-8/98 IEEE. FIG. 2 shows a four leg converter with the input stage (boxes Binl, Bing) and output stage (boxes Bout1, Bout2) completely decoupled.
FIG. 3 shows a four leg AC-DC-AC converter with line and load neutral connected, i.e. the second AC input terminal ACin2 is connected to the second AC output terminal ACout2. This circuit is rather complex, having 4 legs and 4 inductors.
FIG. 4 shows a circuit where the four legs of FIG. 3 has been reduced to 3 legs and 3 inductors and which maintains the ability of independent control of AC output with respect to AC input with some restrictions. As for FIG. 1, one half bridge leg Bc is common for the input/output stage. Here, one half bridge Bin is connected to the first AC input terminal ACin1 via an input inductor Lin1, another half bridge Bout is connected to the first AC output terminal ACout1 via an output inductor Lout1, while the common half bridge Bc is connected to the second AC input terminal ACin2 via a common inductor Lc. As shown in FIG. 4, the second AC input terminal ACin2 is directly connected to the second AC output terminal ACout2, and hence, the common half bridge Bc is considered also to be connected to the second AC input terminal ACin2 via the common inductor Lc. The switches need to be able to block positive voltage and conduct negative current. MOSFETs are therefore suitable having the desired properties, since there is a parasitic body-drain diode capable of conducting current in the negative direction, see FIG. 5.
A disadvantage of having a combination of two MOSFET switches in a half-bridge configuration switching with high frequency is the excessive switching losses coming from the recovery of the slow body-drain diodes. To overcome this problem and allow for high frequency switching, and get all the benefits from high frequency switching in combination with high efficiency one has to prevent the body drain diodes to conduct and allow the current to flow (free-wheel) in another branch.
One object of the present invention is to provide an AC-AC converter device which is achieving high efficiency and still keep the advantage of having a circuit that allows for a phase shift between line input and load output voltage. Another object of the invention is to provide an AC-AC converter device with reduced losses and thus higher efficiency.